The invention relates to pulse-compression techniques, and more particularly to a pulse-compression technique using a fast Fourier transform circuit in a matched filter.
It is known to raise the average transmitted power of a given radar by expanding the pulse length and simultaneously keeping a constant bandwidth so that the range resolution capability of the radar is not reduced. To achieve this, a long pulse containing some sort of a linear frequency modulation or stepwise approximation to a liner frequency modulation is transmitted. Upon reception, the pulse is compressed to permit separation of adjacent range resolution cells.
U.S. Pat. No. 4,237,461 issued to Ben H. Cantrell on Dec. 2, 1980, discloses a digital pulse-expander-compressor which uses a stepwise approximation to a linear frequency modulation.
This device has poor Doppler tolerance. That is, the autocorrelation function or compressed-pulse waveform that is obtained with the device exhibits large range-time grating lobes in the presence of large Doppler shifts on the echo pulses. These large grating lobes are undesirable as they may give rise to false detections and range, if time discrimination is used to calculate range.